Amplifier coupling circuit



Dec. 2, 1941. c. s. ROOT AMPLIFIER COUPLING CIRGUI'I Filed April l 3,'1940 Inventor:

m w R m 5. m s t b m A r s a h H y b Patented Eco. 2, 1941 OFFICEAMPLEIER COUPLING CIRCUIT Charles S. Root, Bridgeport, Conn, assignor toGeneral Electric Company, a corporation of New York Application April13, 1940, Serial No. 329,501

4 Claims.

This invention relates to amplifier coupling circuits and has for anobject to provide an improved amplifier coupling circuit which may bereadily adjusted to be either highly selective or less selective for abroad band of frequencies.

It is also an object of my invention to provide an amplifier couplingcircuit having three cascaded tuned circuits in which the over-allselectivity is adjusted between a high and low value in an improved way.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. 1 illustrates a portion of a radioreceiver embodying my invention; Fig. 2 illustrates a transformerespecially constructed for use in the receiver of Fig. 1, and Fig. 3includes curves showing the over-all selectivity of the circuit shown inFig. 1.

Fig. 1 represents my invention as applied to a radio frequency receiver.A transformer I impresses a radio frequency signal wave from a source,not shown, such as the output circuit of a radio frequency amplifier,upon a multi-grid electron discharge device H which acts as a converterto change the frequency of the incoming wave to a fixed intermediatefrequency. The wave of this intermediate frequency is amplified by asecond electron discharge device I2 and is transmitted to furtherportions of the receiver, not shown, including a detector and an audioreproducing system.

The local oscillation circuits of the converter include a transformer I3and a tuned condenser M in shunt to one winding thereof. The windings ofthis transformer are connected respectively through coupling condensersl5, l6, and through ground to appropriate electrodes of the dischargell. One of these electrodes, which acts as an anode of the localoscillator, is supplied from a source of operating potential, not shown,through a resistor i'l. Another of these electrodes. which acts as acontrol grid of the local oscillator is biased by means of a grid leakresistor is. The screen grids of the device II are supplied from thesame source of operating potential through a resistor i9 and are shuntedto ground through a bypassing condenser 20.

The transformer It! is tuned to resonance with the incoming radiofrequency wave by a condenser 2!, which is mechanically connected withthe tuning condenser I4 in a well known manner for unicontrol, so that,by heterodyne action betweenthe incoming radio frequency wave and thewave generated in the circuit I3, M a wave of fixed intermediatefrequency is produced for any received signal. The secondary of thetransformer II) is connected between the second control grid of the.device H and a source of grid bias potential, not shown, which isshunted to ground through a by-passing condenser 22'. The cathode of thedevice II is connected to ground through a cathode resistor .23 shuntedby a by-passing condenser 24. The anode of the device, H is suppliedfrom a source of potential, not shown, through a resistor 25 and throughan inductance 26, which forms a resonant circuit with .a shunt connectedtuning condenser 21. .A point betweenthe inductance .26 and the resistor25 is icy-passed to ground by a high frequency Joy-passing condenser 28.Heterodyne action in the device U, as explained above, produces :a waveof intermediate frequency to which the tuned circuit 2.6, 21 responds,so that a wave of intermediate frequency appears therein whenever aradio frequency wave is transmitted through the transformer l0.

A coupling circuit for transmitting the intermediate frequency wave fromthe device 1] to the device I2 comprises three cascaded tuned circuitsof which the circuit 26, .21 is the first. The second tuned circuitincludes an inductance 29 magnetically coupled to the inductance 26 anda condenser 36 connected through a switch V3! in shunt .to theinductance 29. The third tuned circuit comprises an inductance 32connected at one end to the control grid of the device l2 and .at theother end through a switch 33 to a suitable source of grid biaspotential, not shown, which is shunted to ground through a by-passingcondenser 34. This third tuned circuit also includes .a tuning condenser35 connected between the control grid of the device I2 and ground. Theinductance 32 is magnetically coupled to the inductance 29. I

As just described and as connected in the figure the three cascadedtuned circuits are .adjusted for maximum selectivity. To attain 'amaximum selectivity characteristic each pair of adjacent coupled tunedcircuits must have about the same degree of coupling as .every otherpair of adjacent tuned circuits in the group. The coupling between thenon-adjacent, or remote, .ones of the cascaded tuned circuits is made aminimum. r

The cathode of the device I2 is connected to ground through a cathoderesistor 36 shunted by a by-passing condenser 31. The screen grid of thedevice I2 is supplied from a source of potential, not shown, shunted toground through a by-passing condenser 38. The anode of the device isconnected through a tuned circuit 39, 40 identical with the tunedcircuit 26, 21 to a source of potential, not shown, which is shunted toground by a by-passing condenser 4|. The tuned circuit 39, 40 is thefirst of three additional cascaded tuned circuits forming a couplingarrangement similar to that between the devices H and I2. It need bedescribed no further, since it is understood that it may be constructedand operated in an identical manner to the first coupling arrangement.

To adjust the coupling device for much less selectivity, so that a muchwider band of frequencies may be transmitted, an inductance 42 isprovided which is magnetically coupled closely to the inductance 26. Oneend of this inductance 42 is connected to that end of the inductance 32remote from the control grid of the device |2. The switch 33 isconnected so that, in its alternative position it provides a seriesconnection for high frequency currents through the inductance 32, theinductance 42 and through the by-passing condenser 34 to ground. Uponoperation of the switch 33 to connect the inductances 32 and 42 inseries, there is produced magnetic overcoupling between the devices IIand I2, which provides a greatly broadened frequency transmissioncharacteristic.

The inductance 42, providing over-coupling be- 7 tween the remote tunedcircuits 26, 21 and 32, 35, produces a desirable lessened selectivitycharacteristic for the coupling system comprising three cascaded tunedcircuits. If over-coupling be produced between only one pair of adjacenttuned circuits of such a group, the selectivity characteristics for thesystem assumes a shape such that waves of some desired frequencies aretransmitted with maximum amplitudes and waves of other desiredfrequencies with much less 1 amplitudes. My invention avoids such aselectivity characteristic and achieves a desirable characteristichaving substantially uniform transmission over a relatively wide rangeof frequencies with the use of only one device for producingover-coupling.

It is preferred that the switches 3| and 33 be mechanically connectedtogether for simultaneous operation so that, when the inductances 32 and42 are connected in series to provide overcoupling, the tuned circuit29, is broken so as to be inoperative. Thus, in the upper position ofthe switches 3| and 33 the coupling arrangement comprises three looselycoupled cascaded tuned circuits, while in the lower position of theswitch the coupling arrangement comprises only two relatively tightlycoupled cascaded circuits.

Although the inductance 42 is preferred, as a means of providingover-coupling between the tuned circuit 26, 21 and the tuned circuit 32,a condenser 43 may be connected from the upper endof the tuned circuit26, 2! through a switch 44 to the upper end of thetuned circuit 32, 35.The magnitude of the condenser is made such as to produce a suitableamount of over-coupling such as was produced by the inductance 42. Itis, of course, preferred, if this alternative scheme be used, to coupletogether mechanically the switches 3| and 44 so that they operatesimultaneously, thus producing the same results as in the otherarrangement. Both types of coupling may be employed, if it be desired.

In Fig, 2, a transformer comprising inductances 26, 42, 29 and 32 isshown. This figure illustrates the manner in which this transformer maybe constructed. The inductances are wound as illustrated upon acylindrical coil form 45 made of insulating material. At that end of thecoil form 45 nearest the inductance 32, three connectors 46, 41, 48, arefastened. These connectors are adapted to cooperate with a widely usedtype of gang switch, which may comprise the switches 3| and 33.Disk-like fiber spreaders 49 and 5|] fit snugly around the coil formrespectively between the inductances 42 and 29, and between theinductances 29 and 32 in order to prevent connecting wires from touchingor 1ying near inductances which they cross. These spreaders aid inreducing unwanted capacity effects.

It will be observed that coils 26 and 32 are spaced widely apart so asto reduce direct coupling between them, and that these coils are aboutequally spaced from intermediate coil 29 to produce coupling betweencoils 26 and 29 equal to that between coils 29 and 32. The coil 42 ispositionednear coil 26 and must be made quite large to produce as muchcoupling between the remote tuned circuits as exists therebetween byreason of coil 29. Actually it is made larger than this to obtain anovercoupled fiat resonance curve for best fidelity of high notes whencoil 29 is open-circuited. Of course if the circuit of coil 29 wereallowed to remain closed, this circuit would act as an absorptioncircuit and produce an undesirable double humped resonance curve.

The curves of Fig. 3 show the relation between the difference betweenthe side band frequency and the carrier frequency, plotted as abscissa,and the ratio of the output of the intermediate frequency channel of areceiver embodying my invention to the input to that channel, plotted asordinate. The deviation of the side band frequency from carrier isplotted in kilocycles. Curve 5| shows the selectivity of the completeintermediate frequency channel of a receiver such as is illustrated inpart by Fig. 1, wherein the switches 3| and 33 are in the upperposition. It may be seen that selectivity is at a maximum, so as totransmit the usual audio frequencies tolerably well. Curve 52illustrates the over-all selectivity of the same intermediate frequencychannel when the switches 3| and 33 are in their lower positions. Theselectivity of the re-- ceiver has been reduced greatly and thefrequency band transmitted has been greatly broadened so as to providemuch more faithful reception of audio frequencies, or in other words toprovide for reception of much higher audio frequencies.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto, sincedifferent modifications may be made both in the circuit arrangement andinstrumentalities employed, and I aim by the appended claims to coverany such modifications as fall within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a coupling device for transmitting high frequency waves, thecombination of at least three cascaded tuned circuits having apredetermined amount of coupling between adjacent ones of said tunedcircuits and a minimum amount of coupling between remote ones thereof,means for producing a greater amount of coupling between said remoteones of said cascaded tuned circuits, means to open-circuit at least oneof said tuned circuits intermediate said remote ones of said tunedcircuits, and means for operating said coupling increasing means andsaid open circuiting means simultaneously to change the selectivity ofsaid tuned circuits.

2. In a coupling device for transmitting high frequency waves, thecombination of at least three cascaded tuned circuits having apredetermined amount of magnetic coupling between adjacent ones of saidtuned circuits and a minimum amount of magnetic coupling between remoteones thereof, means for producing a greater amount of coupling betweensaid remote ones of said cascaded tuned circuits, means to open-circuitat least one of said tuned circuits intermediate said remote ones ofsaid tuned circuits, and means for operating said coupling increasingmeans and said open circuiting means simultaneously to change theselectivity of said tuned circuits.

3. In a coupling device for transmitting high frequency waves, thecombination of at least three cascaded tuned circuits having apredetermined amount of coupling between adjacent ones of said tunedcircuits and a minimum amount of magnetic coupling between remote onesthereof, means for adding capacitive coupling between said remote onesof said cascaded tuned circuits, means to open-circuit at least one ofsaid tuned circuits intermediate said remote ones of said tunedcircuits, and means for operating said coupling adding means and saidopen circuiting means simultaneously to change the selectivity of saidtuned circuits.

4. A coupling device for transmitting high frequency waves, comprisingat least three cascaded tuned circuits having a predetermined amount ofcoupling between adjacent ones of said tuned circuits and a minimumamount of coupling between remote ones thereof, energy transfer betweenthe remote tuned circuits being provided within a band of frequenciesthrough an intermediate one of said tuned circuits, means for opencircuiting at least said one intermediate tuned circuit thereby toreduce energy transfer between said remote tuned circuits within saidband of frequencies, means for producing a greater amount of couplingbetween said remote tuned circuits, said last means being substantiallyindependent of frequency over a frequency range substantially wider thansaid band of frequencies, and means for operating saidcouplingincreasing means and said open circuiting means simultaneously to changethe selectivity of said tuned circuits.

CHARLES S. ROOT.

